Device for hot-dip coating a metal bar

ABSTRACT

The invention relates to a device for hot-dip coating a metal bar ( 1 ), particularly a steel strip, in which the metal bar ( 1 ) is vertically directed through a container ( 3 ) receiving the molten coating metal ( 2 ) and a directing channel ( 4 ) that is arranged upstream thereof. Said device comprises at least two inductors ( 5 ) which are disposed on both sides of the metal bar ( 1 ) in the zone of the directing channel ( 4 ) and generate an electromagnetic field for retaining the coating metal ( 2 ) within the container ( 3 ). In order to better control the coating process, the inventive device is characterized by a sealing means ( 7, 7 ′) which is arranged above the directing channel ( 4 ) in the bottom area ( 6 ) of the container ( 3 ) and alternatively releases or interrupts the flow of molten coating metal ( 2 ) to the metal bar ( 1 ) and/or the directing channel ( 4 ).

The invention concerns a device for hot dip coating a metal strand,especially a steel strip, in which the metal strand is passed verticallythrough a coating tank that contains the molten coating metal andthrough a guide channel upstream of the coating tank, with at least twoinductors for generating an electromagnetic field, which are installedon both sides of the metal strand in the area of the guide channel inorder to keep the coating metal in the coating tank.

Conventional metal hot dip coating installations for metal strip have ahigh-maintenance part, namely, the coating tank and the fittings itcontains. Before being coated, the surfaces of the metal strip must becleaned of oxide residues and activated for bonding with the coatingmetal. For this reason, the strip surfaces are subjected to heattreatments in a reducing atmosphere before the coating operation iscarried out. Since the oxide coatings are first removed by chemical orabrasive methods, the reducing heat treatment process activates thesurfaces, so that after the heat treatment, they are present in a puremetallic state.

However, this activation of the strip surfaces increases their affinityfor the surrounding atmospheric oxygen. To prevent the surface of thestrip from being reexposed to atmospheric oxygen before the coatingprocess, the strip is introduced into the hot dip coating bath fromabove in an immersion snout. Since the coating metal is present in themolten state, and since one would like to utilize gravity together withblowing devices to adjust the coating thickness, but the subsequentprocesses prohibit strip contact until the coating metal has completelysolidified, the strip must be deflected in the vertical direction in thecoating tank. This is accomplished with a roller that runs in the moltenmetal. This roller is subject to strong wear by the molten coating metaland is the cause of shutdowns and thus loss of production.

The desired low coating thicknesses of the coating metal, which vary inthe micrometer range, place high demands on the quality of the stripsurface. This means that the surfaces of the strip-guiding rollers mustalso be of high quality. Problems with these surfaces generally lead todefects in the surface of the strip. This is a further cause of frequentplant shutdowns.

To avoid the problems associated with rollers running in the moltencoating metal, approaches have been proposed, in which a coating tank isused that is open at the bottom and has a guide channel in its lowersection for guiding the strip vertically upward, and in which anelectromagnetic seal is used to seal the open bottom of the coatingtank. The production of the electromagnetic seal involves the use ofelectromagnetic inductors, which operate with electromagneticalternating or traveling fields that seal the coating tank at the bottomby means of a repelling, pumping, or constricting effect.

A solution of this type is described, for example, in EP 0 673 444 B1.The solution described in WO 96/03,533 and the solution described in JP50[1975]-86,446 also provide for an electromagnetic seal for sealing thecoating tank at the bottom.

In this regard, guaranteeing the tightness of the seal of the coatingtank guide channel, which is open at the bottom, is an important anddifficult problem, above all in an emergency situation in which theelectromagnetic seal may fail due to a power outage. Variouspossibilities for dealing with this situation have been disclosed in theprior art.

EP 0 630 421 B1 provides for a constriction below the guide channel,from which a pipe leads to a reservoir for molten coating metal. Thisdocument does not disclose detailed information on the design of thisdevice, which is referred to as a reflux barrier.

JP 2000-273,602 discloses a collecting tank below the guide channel,which is intended to collect coating metal that runs down through theguide channel. The coating metal is conveyed to a tank, from which it ispumped back into the coating tank by a pump. Here again, no definite andspecific information is provided about how the coating metal that runsout is to be collected.

EP 0 855 450 B1 deals in greater detail with the question of how thetightness of the lower region of the guide channel can be guaranteed. Itdiscloses various alternative solutions for guaranteeing this. In oneembodiment, two slides installed on either side of the metal strand canbe moved up to the surface of the metal strand perpendicularly to themetal strand. The slides act as plugs and, when necessary, are held incontact with the metal strand to prevent molten metal from escaping downthrough the guide channel. However, relatively expensive automaticcontrol of the slides is necessary to guarantee their function. Inanother embodiment, a belt conveyor is used, which conveys the escapingcoating metal from the area below the guide channel to a collectingtank. However, this solution is very expensive and entails the risk thatthe belt will become clogged with coating metal in the course of timeand thus will no longer be able to function properly. A thirdalternative solution for preventing the escape of molten coating metalinvolves the use of a gas jet system. A stream of gas is directed at theguide channel from below, which is intended to force the escapingcoating metal back up and thus seal the opening of the guide channel atthe bottom. This solution is also expensive and has limited practicalsuitability.

FR 2 798 396 A discloses a hot dip coating installation in which abarrier is arranged in the bottom area of the coating tank at thetransition to the guide channel. This is intended to keep molten metalin the coating channel from entering the guide channel. To this end, thebarrier is equipped with walls or deflectors that are designed forfavorable flow. However, the barrier disclosed in this document is notsuitable for keeping the molten coating metal out of the area of theguide channel in emergency situations. Similarly, the coating processcannot be influenced with this barrier.

EP 0 855 450 A1 describes a solution in which a temporary seal betweenthe molten metal in the coating tank and the guide channel is producedwith a seal that consists of a fusible material whose melting point isno higher than that of the coating metal. After this seal has melted,the fluid connection between the molten metal in the coating tank andthe guide channel is established.

Therefore, the objective of the invention is to develop a device for hotdip coating of a metal strand, with which it is possible to conduct thecoating process in an optimum way and also by simple means to guaranteereliable operation of the installation in critical operating states, forexample, if the inductor power supply is interrupted.

The solution of this problem in accordance with the invention ischaracterized by sealing means arranged above the guide channel in thebottom area of the coating tank for selectively releasing orinterrupting the flow of molten coating metal to the metal strand and/orto the guide channel, such that the sealing means are designed as a weirthat can be moved relative to the bottom area of the coating tank.

In accordance with the invention, the flow of the coating metal,especially to the guide channel, can be selectively released orinterrupted, so that, especially in the case of a disruption of theoperation, there is no danger that molten metal can escape from thecoating installation through the guide channel.

This design makes it possible to avoid damage of the coatinginstallation and economic loss in the event of such a disruption.

In accordance with one embodiment, the weir has two interacting parts,each of which can be moved perpendicularly to the surface of the metalstrand. Alternatively or additionally, it can be provided that the weircan be moved in the direction of conveyance of the metal strand.

In the latter case, it can be provided that the weir is formed as asingle piece and has the form of a box. This makes it possible both toproduce the weir inexpensively and to guarantee the operationalsuitability of the device in an especially simple way.

It is advantageous for the weir to have covering means in its upper endregion that face away from the bottom area of the coating tank. Thesecovering means make it possible to quiet the coating bath, into whichturbulence is introduced by the electromagnetic excitation produced bythe inductors. In one embodiment, the covering means are designed aswall sections that extend parallel to the bottom area of the coatingtank. In another embodiment, the covering means are designed as a platethat has a slot-like opening for the passage of the metal strand.

The sealing means, especially the weir, are preferably connected withmanual, pneumatic or hydraulic operating mechanisms. In this regard, theoperating mechanisms can be connected with an installation controlsystem, which effects the release or interruption of the flow of moltencoating metal to the metal strand and/or to the guide channel.

Embodiments of the invention are illustrated in the drawings.

FIG. 1 shows a schematic section through a hot dip coating device with ametal strand being guided through it.

FIG. 2 shows a perspective view of a weir constructed from two pieces.

FIG. 3 shows a perspective view of a weir constructed as a single piece.

FIG. 4 shows a schematic section through the hot dip coating device witha weir that is constructed from two pieces and equipped with coveringmeans.

FIG. 5 shows a perspective view of a weir that is constructed as asingle piece and equipped with covering means.

FIG. 1 shows a schematic section through a hot dip coating device with ametal strand 1 being guided through it.

The device has a coating tank 3, which is filled with molten coatingmetal 2. The coating metal 2 can be, for example, zinc or aluminum. Themetal strand 1 in the form of a steel strip passes vertically upwardthrough the coating tank 3 in conveying direction R. It should be notedat this point that it is also basically possible for the metal strand 1to pass through the coating tank 3 from top to bottom. To allow passageof the metal strand 1 through the coating tank 3, the latter is open atthe bottom, where a guide channel 4 is located.

To prevent the molten coating metal 2 from flowing out at the bottomthrough the guide channel 4, two electromagnetic inductors 5 are locatedon either side of the metal strand 1. The electromagnetic inductors 5generate a magnetic field, which counteracts the weight of the coatingmetal 2 and thus seals the guide channel 4 at the bottom.

The inductors 5 are two alternating-field or traveling-field inductorsinstalled opposite each other. They are operated in a frequency range of2 Hz to 10 kHz and create an electromagnetic transverse fieldperpendicular to the conveying direction R. The preferred frequencyrange for single-phase systems (alternating-field inductors) is 2 kHz to10 kHz, and the preferred frequency range for polyphase systems (e.g.,traveling-field inductors) is 2 Hz to 2 kHz.

In the embodiment shown in FIG. 1, a two-part sealing means 7, and 7′ inthe form of a weir is installed in the bottom area (6) of the coatingtank 3. The two parts 7, 7′ of the weir can be moved parallel to thebottom of the coating tank 3 in the direction of the double arrow. Thismovement is accomplished with operating mechanisms 11, which areillustrated here only schematically as piston-cylinder units; any othertype of operating mechanism 11 can be used in the same way.

In the present case, the weir 7 and 7′ is constructed as a two-part box.The two halves 7 and 7′ can interact in such a way that they partitionoff the region of the guide channel 4 in the bottom area 6 of thecoating tank 3. This situation is shown in FIG. 1. Consequently, thecoating metal 2 cannot reach the guide channel 4 or the metal strand 1.This closed position of the weir 7 and 7′ is important especially in twooperating states:

First, this position is assumed before the coating installation isbrought to full speed. The metal strand 1 is then moving upward inconveying direction R (without coating metal 2 being able to reach it),and the inductors 5 are activated. Only then are the two parts 7 and 7′of the weir moved away from the metal strand 1 in the direction of thedouble arrow, so that coating metal 2 can pass through the opening boxand reach the metal strand 1 and the area of the guide channel 4. Sincethe inductors 5 are activated, the coating metal 2 cannot escapedownward through the guide channel 4. Therefore, the weir 7, 7′initially encloses the guide channel 4, which is open at the bottom, andthus the metal strand 1 passing through the guide channel up to anoptimized height above the bottom area 6 of the coating tank 3, so thatno coating metal 2 can flow towards the guide channel 4. When thecoating process is begun, the weir 7, 7′ is then opened, so that thecoating metal 2 can flow, in a way that is optimized with respect totime and volume, to the metal strand 1 and thus into the guide channel4, which, however, is now electromagnetically sealed by the inductors 5.

Second, the weir 7, 7′ is also important when a power failure occurs,and the inductors 5 (e.g., until an emergency power system starts up)are no longer able to perform their function, namely, to seal the guidechannel at the bottom by the electromagnetic field they generate. Inthis case, the two parts 7, 7′ of the weir are moved towards the metalstrand 1 in the direction of the double arrow until they touch and formthe box-shaped covering around the metal strand 1. Consequently, coatingmetal 2 can no longer reach the metal strand 1 and the guide channel 4,i.e., the guide channel 4 is now mechanically sealed. This preventscoating metal 2 from flowing down and out of the guide channel 4.

In FIG. 2, the weir 7, 7′ is shown again in a perspective view in itsclosed state. The double arrows indicate the direction in which the twoparts 7, 7′ of the weir can be moved relative to the conveying directionR of the metal strand 1. This movement is effected by the operatingmechanisms 11 (see FIG. 1). The drawing shows that there is an openingfor the passage of the metal strand 1 in the bottom of the weir 7, 7′.However, in the illustrated closed position of the weir 7, 7′, it isensured that no coating metal 2 can reach the metal strand 1 and theguide channel 4.

Since the weir 7, 7′ is exposed to the coating metal 2, it isadvantageous for stable and reliable operation of the weir 7, 71 if itconsists of as few individual parts as possible. Whereas the embodimentshown in FIGS. 1 and 2 consists of a two-part weir 7, 7′, FIG. 3 showsthat the weir 7 can also be constructed as a single piece. In this case,in its closed state, the box-shaped weir 7 rests on the bottom 6 of thecoating tank 3 and thus seals the guide channel 4. The weir 7 is openedby moving it vertically upward, i.e., in conveying direction R, byoperating mechanisms 11.

To carry out a coating process for producing a qualitatively high-gradecoated metal strand, it is advantageous if care is taken to ensure thatthe surface of the coating bath remains as calm as possible. This is notinherently guaranteed, because the electromagnetic inductors 5 induceflow in the coating metal 2 by the magnetic fields that they generate.

In the embodiment shown in FIG. 4, to quiet the surface of the coatingbath, covering means 9 are provided in the end region 8 of the weir 7,7′, which ensure that the currents induced by the inductors 5 cannotspread farther in the direction of the surface of the bath.

The turbulence of the molten coating metal 2 produced in the guidechannel 4 and in the coating tank 3 by the electromagnetic seal can beshielded by the design of the weir 7, 7′ and especially by the cover 9.

When the weir 7 is constructed as a single piece, the possibility shownin FIG. 5 is realized: In this case, the weir 7 is provided with anopening 10 at the top to allow the metal strand 1 to pass through. Thecurrents induced in the coating metal 2 by the inductors 5 are stoppedhere by the covering means 9, which produce almost complete isolation ofthe interior of the weir 7 from the rest of the coating bath. Thisdesign makes it possible to achieve optimum quieting of the bath surfaceand thus to ensure a quality coating.

In the event of an operational disruption and especially in the event offailure of the electromagnetic inductors 5, the weir 7 is closed by theoperating mechanisms 11, so that there is no danger of the coating metal2 escaping from the coating tank 3.

LIST OF REFERENCE SYMBOLS

-   1 metal strand (steel strip)-   2 coating metal-   3 coating tank-   4 guide channel-   5 inductor-   6 bottom area of the coating tank-   7 sealing means-   7′ sealing means-   8 end region of the sealing means-   9 covering means-   10 opening-   11 operating mechanism-   R conveying direction

1. Device for hot dip coating a metal strand (1), said device comprisinga tank (3) that contains the molten coating metal (2) and an upstreamguide channel (4) such that a metal strand (1) can be guided verticallythrough the guide channel (4) and the tank (3) thereby coating the metalstrand (1) with at least two inductors (5) for generating anelectromagnetic field, which are installed on both sides of the metalstrand (1) in the area of the guide channel (4) in order to keep thecoating metal (2) in the coating tank (3), comprising sealing means (7,7′) arranged above the guide channel (4) in the bottom area (6) of thecoating tank (3), wherein the sealing means (7, 7′) can be positioned ina first position, in which the molten coating metal (2) can be releasedto flow to the metal strand (1) and/or to the guide channel (4), and ina second position, in which the flow of molten coating metal (2) to themetal strand (1) and/or to the guide channel (4) can be interrupted, andwherein the sealing means (7, 7′) are designed as a weir that can bemoved relative to the bottom area (6) of the coating tank (3), whereinthe weir has two interacting parts (7, 7′), and further comprisingoperating mechanisms (11) operative to move the parts (7, 7′)perpendicular to the surface of the metal strand (1) and to thedirection of conveyance of the metal strand and parallel to the bottomwall of the coating tank (3).
 2. Device for hot dip coating a metalstrand (1), said device comprising a tank (3) that contains the moltencoating metal (2) and an upstream guide channel (4) such that a metalstrand (1) can be guided vertically through the guide channel (4) andthe tank (3) thereby coating the metal strand (1), with at least twoinductors (5) for generating an electromagnetic field, which areinstalled on both sides of the metal strand (1) in the area of the guidechannel (4) in order to keep the coating metal (2) in the coating tank(3), comprising sealing means (7, 7′) arranged above the guide channel(4) in the bottom area (6) of the coating tank (3), wherein the sealingmeans (7, 7′) can be positioned in a first position, in which the moltencoating metal (2) can be released to flow to the metal strand (1) and/orto the guide channel (4), and in a second position, in which the flow ofmolten coating metal (2) to the metal strand (1) and/or to the guidechannel (4) can be interrupted, and wherein the sealing means (7, 71)are designed as a weir that can be moved relative to the bottom area (6)of the coating tank (3), wherein the weir (7, 7′) has covering means (9)in its upper end region (8) that face away from the bottom area (6) ofthe coating tank (3), and further comprising operating mechanisms (11)operative to move the weir (7, 7′) perpendicular to the direction ofconveyance of the metal strand (1) and parallel to the bottom wall ofthe coating tank (3).
 3. Device in accordance with claim 2, wherein theweir can be moved in the direction of conveyance (R) of the metal strand(1).
 4. Device in accordance with claim 2, wherein the covering means(9) are designed as wall sections that extend parallel to the bottomarea (6) of the coating tank (3).
 5. Device in accordance with claim 2,wherein the covering means (9) are designed as a plate that has aslot-like opening (10) for the passage of the metal strand (1). 6.Device for hot dip coating a metal strand (1), said device comprising atank (3) that contains the molten coating metal (2) and an upstreamguide channel (4) such that a metal strand (1) can be guided verticallythrough the guide channel (4) and the tank (3) thereby coating the metalstrand (1), with at least two inductors (5) for generating anelectromagnetic field, which are installed on both sides of the metalstrand (1) in the area of the guide channel (4) in order to keep thecoating metal (2) in the coating tank (3), comprising sealing means (7,7′) arranged above the guide channel (4) in the bottom area (6) of thecoating tank (3), wherein the sealing means (7, 7′) can be positioned ina first position, in which the molten coating metal (2) can be releasedto flow to the metal strand (1) and/or to the guide channel (4), and ina second position, in which the flow of molten coating metal (2) to themetal strand (1) and/or to the guide channel (4) can be interrupted, andwherein the sealing means (7, 7′) are designed as a weir that can bemoved relative to the bottom area (6) of the coating tank (3), whereinthe sealing means (7, 7′) are connected with manual, pneumatic orhydraulic operating mechanisms (11), wherein the weir has twointeracting parts (7, 7′), each of which can be moved by the operatingmechanisms (11) perpendicular to the surface of the metal strand (1),and parallel to the bottom wall of the coating tank (3) as well as upand down in the coating tank (3) along the direction of conveyance ofthe metal strand (1).
 7. Device in accordance with claim 6, wherein theoperating mechanisms (11) are connected with an installation controlsystem, which effects the release or interruption of the flow of moltencoating metal (2) to the metal strand (1) and/or to the guide channel(4).